Tailored deep-eutectic solvent method to enable 3D porous iron fluoride bricks for conversion-type lithium batteries

Conversion-type fluoride cathode can provide considerable energy density for Li batteries, however its scalable and facile synthesis strategies are still lacking. Here, a novel Fe-based deep eutectic solvent composed of nitrite and methylsulfonylmethane is proposed as both the reaction medium and precursor to synthesize O-doped FeF3 porous bricks. This method is cheaper, safe, mildly operable, environmentally friendly and recyclable for non-fluorinated metal cations. The homogenization of charge and mass transport in cathode network effectively mitigates the volume extrusion and electrode coarsening even for the micro-sized monolithic particles. The Co-solvation modulated fluoride cathode delivers high reversible capacity in a wide temperature range (486 and 235 mA h g−1 at 25 °C and –20 °C respectively), excellent rate performance (312 mA h g−1 at 1000 mA g−1), corresponding to an energy density as high as 672.1 W h kg−1 under a power density of 2154.3 W kg−1. The successful operation of fluoride pouch-cell with a capacity exceeding 450 mA h g−1 (even under thin Li foil and lean electrolyte conditions) indicates its potentiality of commercial application.